ensure set_variable only takes into account GET variables, not SET

Author: lovasoa

tests/sql_test_files/it_works_set_variable_null.sql

@@ -1,5 +1,5 @@
-set url = sqlpage.set_variable('x', null);
 set path = sqlpage.path();
+set url = sqlpage.set_variable('x', null);
 select 'text' as component,
     case
         when $url = $path THEN 'It works !'